DRAMs (Dynamic Random Access Memories) are highly integrated semiconductor memory circuits. DRAM devices are well-known in the art.
This invention concerns a novel construction of a DRAM for use in video imaging. Presently, the video imaging process primarily employs a special kind of integrated circuit (IC) memory device known as a video random access memory (VRAM). VRAMs incorporate more circuitry than is customarily employed in conventional DRAMs to enable faster memory speeds which are required by such video imaging processes. While VRAMs are widely used, DRAMs also have a market in the video arena, particularly for those customers who do not require the faster access speeds. DRAMs have adequate speeds and are less expensive to use. Accordingly, there is a performance/cost tradeoff between alternative use of VRAMs and DRAMs in the video imaging market, with DRAMs having lower performance and lower cost than the VRAMs. It would be advantageous to employ DRAMs to perform those tasks in video image processing that do not require high speeds.
One particular operation which may be performed using a VRAM is a "row copy" function. This function allows the identical data to be written in multiple rows of memory cells. In video imaging, this function is useful when creating a solid block area on a video screen that requires a repetition of information. For example, in word processing screen displays, the background images often comprise an entire screen of a single color, such as blue, black or green. The written text is then produced in another color on top of this background image. The solid block background area is created by the row copy operation. Each bit of memory information is the same for each column and row in the memory array assigned to that block of color.
The row copy function is a highly repetitious task that does not require fast access speeds and thus, does not require the high performance of VRAMs. It would be desirable if this function could be performed by the less expensive DRAMs so that the VRAMs could be freed to perform higher speed operations. The row copy function is implemented in firmware utilizing transfer modes only available on VRAMs. Although uniform backgrounds are used quite often in today's programs, there is no standard software/firmware designed to implement the repetitive operation in VRAMs. Separate software programs are written for the wide variety of VRAMs in use today. It would be desirable to have a standard, one cycle row copy mode on VRAMs to avoid the need for a firmware implementation. Unfortunately, firmware programs cannot be ported to the DRAMs.
An unrelated problem concerns graphic cards which employ only DRAMs. To create a background by performing the row copy function, these cards need other circuitry on the printed circuit board (PCB) to perform this operation. It would be desirable if the DRAMs could perform this function. The PCB could then be made less expensive. Moreover, additional space on the PCB that was created by eliminating the traditional row copy circuitry would be available for other features to be added to the card. Alternatively, a DRAM-implemented row copy would allow the same features as currently offered, but on a smaller PCB.
Another problem concerns testing DRAM and related devices. At various stages of fabrication, DRAM devices are tested by writing and reading a test pattern of data to the memory cells. Common test patterns include all binary "1"s or "0"s, alternating "1"s and "0"s, or a checkerboard of "1"s and "0"s. Prior art DRAMs use block and flash write operations which permit the writing of all binary "1"s or "0"s to the memory array. However, these modes are not included on all DRAMs and there is no technique for handling test patterns of mixed "1"s and "0"s. It would be desirable to design a DRAM which quickly produced any desired test pattern.
This invention concerns a DRAM device which overcomes the above drawbacks.